1. Field of the Invention
The present invention relates to a combustion detection device for use in an internal combustion engine.
2. Description of the Conventional Art
Description will be given below of an example of a conventional combustion detect device of this type with reference to FIG. 24. In FIG. 24, reference numeral 1 designates an ignition coil which includes a primary winding 1a and a secondary winding 1b; 2, a power transistor which is connected to the primary winding 1a and is used to cut off a primary current flowing through the primary winding 1a; 3, an ignition plug which is connected to the secondary winding 1b and, when a high voltage for ignition is applied thereto, is able to ignite a gas mixture of an internal combustion engine (not shown); 4, a power supply which is to apply a bias voltage of a positive polarity to the ignition plug 3 as bias means; 5, a resistor which is connected to the power supply 4 in series and is used to convert an ion current into a voltage; and 6, an output terminal which is used to detect the voltage across the resistor 5 and cooperates with the resistor 5 in forming ion current detect means.
In the above-mentioned structure, at the ignition timing of the internal combustion engine, the power transistor 2 is turned off to thereby cut off the primary current of the primary winding 1a, then there is produced an ignition high voltage of a negative polarity in the secondary winding 1b to thereby cause discharge across the electrode of the ignition plug 3, so that the gas mixture of the internal combustion engine can be ignited.
In this operation, with the combustion of the gas mixture, there is produced ionization, so that ions are generated. Here, the electrode of the ignition plug 3, after the abovementioned discharge, acts as an electrode for detecting an ion current and an ion current flows due to the movement of electrons caused by the bias of a positive polarity by the power supply 4. Therefore, by detecting a voltage across the resistor 5 due to the ion current at the output terminal 6, the combustion of the gas mixture can be confirmed.
However, in the above-mentioned conventional combustion detection device, when the power transistor 2 is turned on to thereby allow the primary winding la to conduct, there is generated a high voltage of 1-2 KV in the secondary winding 1b. For this reason, in the case of an ignition coil 1 of a plug-top type being used, the high voltage would cause the ignition plug 3 to discharge before its normal ignition time, so that a premature ignition can occur.